1. Field of the Invention
The present invention relates to an optical filter which absorbs a part of optical signals passing through an optical path and restricts the intensity of transmitted light, and more particularly relates to a configuration of a multilayered optical filter for decreasing a reflectance.
2. Description of the Related Art
Generally, an optimization of a configuration of a multilayer film composing an optical filter can restrict the intensity of transmitted light by absorbing a part of optical signals and thus an optical filter having a predetermined transmittance is obtainable. FIG. 4A shows a configuration of a conventional multilayered optical filter, and FIG. 4B shows a thickness and a refractive index of each layer in the configuration shown in FIG. 4A. The optical filter includes TiO.sub.2 layers having a relatively high refractive index (n.apprxeq.2.5) and SiO.sub.2 layers having a relatively low refractive index (n.apprxeq.1.45), in a thickness of .lambda./5 to .lambda./10, alternately deposited several times between a substrate 1a and a substrate 1b, and also includes a metal film 2 having a lower refractive index (n.apprxeq.0.95) and a thickness of several to several ten nanometers deposited between two adjacent SiO.sub.2 layers, for the purpose of achieving a transmittance of 30% to 40% for light of .lambda.=780 nm passing through a typical optical path.
FIG. 5 shows a reflectance R of the optical filter shown in FIG. 4A at a wavelength .lambda. and FIG. 6 shows a transmittance T against the same. As shown in FIG. 5, although the reflectance R reaches a minimum of approximately 15% when the wavelength .lambda. is nearly equal to 730 nm, it increases at a given wavelength .lambda. of 780 nm. Also, as shown in FIG. 6, although the transmittance reaches a maximum of approximately 35% when a wavelength .lambda. is nearly equal to 720 nm, it slightly decreases at a given wavelength .lambda. of 780 nm.
In such an optical filter, when the reflectance increases, the reflected light adversely affects signal light. As mentioned above, the conventional optical filter has a minimum reflectance R of approximately 15% and cannot achieve a low reflectance, for example, 3% or less.